Biocompatible hemoperfusion, the technique of passing blood through an extracorporeal adsorbent column for the purpose of removing diverse waste metabolites and other toxic substances has become a widely used, safe, life-saving extracorporeal blood detoxification treatment of choice. However, as the biochemistry of major organ failure has become better understood, and as the toxic sequellae (and enormous therapeutic potential) of ever-more-complex chemotherapies have been revealed, the need to selectively and efficiently remove and/or modify specific target substances in the blood has become acute.
While the need for such an enhanced fixed-bed bioreactor has become apparent, the means of providing such a device have remained elusive until the present invention. Indeed, a bioreactor suitable for hemoperfusion presents formidable requirements.
The device must provide exquisite biocompatibility (sterility, antithrombogenicity, etc.) while quickly, efficaciously, and safely removing and/or modifying the target substance and/or reaction product.
Previous attempts to design a suitable blood-perfuseable target bioreactor have been unsuccessful essentially because of two major failings: (1) the devices were insufficiently biocompatible (even when such matters as thromboresistance and damage to formed blood elements were considered at all), and (2) the reactants either did not work or dangerous reactants or reaction products were not contained within the device.
Such previous attempts generally envision a scheme wherein blood is pumped through a fixed polymer bed loaded with the bioreactant. Unfortunately, in practice, this approach has not worked, either because when the reactant is immobilized the polymer prevents reaction of the immobilized bioreactant, or because the polymer is inappropriately porous ("leaky"), and the reactant and/or reaction product leaches out, dangerously releasing hazardous substances into the general circulation. Were these difficulties and others to be overcome, a valuable new extracorporeal therapy could be realized.
Accordingly, it is a primary object of this invention to provide a means of retaining a wide variety of therapeutic reactants and reaction products within a fixed biocompatible hemoperfusion columnar bed while confining full, efficacious, and safe reaction within the column for the purpose of curing disease and sustaining life. It is a further object of this invention to selectively sequester with enhanced efficiency certain target toxic substances and reaction products. It is a still further object of this invention to provide a bioreactor which can be easily and safely sterilized and manufactured.
These and other objects and benefits of the invention will be more fully understood as the invention is set forth and when distinguished from prior art.
Hemoperfusion became a widely-used, life-saving procedure when it was shown in U.S. Pat. No. 4,048,064 to Clark, which patent is incorporated herein by reference, that heparin could be entrapped in a hydrogel, thereby rendering antithrombogenicity and other desirable biocompatible characteristics. Entrapment is a suitable technique for heparin, because heparin activity does not depend upon chemical reaction. This entrapment technique is unsuitable for bioreactants, however, because activity will be impeded in proportion to the extent that the reactant is retained by this entrapment technique which relies on a polymer barrier that has pores smaller than the bioreactant. On the other hand, if a conventional hydrogel is made more porous in order to increase activity, then the bioreactant will escape. It is therefore necessary to provide a means of producing an extremely porous hydrogel which exhibits a powerful avidity for the bioreactant.
In this way, large organic molecules can freely perfuse the gel in order to react with the bioreactant retained by the gel which would be impossible with a prior art gel which simply entraps on the basis of size, because large molecules cannot pass the pore-size "barrier."
The principle of operation of the invention is not complex:
Selected bioreactant molecules are suitably retained for reaction within a porous hydrogel polymer matrix by reversibly quenching the reactant propensity with a displaceable surrogate and polymerizing an insoluble cross-linked hydrogel in such manner that the hydrogel polymer replicates the shape of the bioreactant molecule. This replicated phantom shape in the polymer then exhibits profound dispersion-force affinity for the bioreactant.
The quenching surrogate latentiates the reactive propensity of the bioreactant. The bioreactant is "quenched," because reaction with the monomer is prevented by the "surrogate" which alters the relative solvations of the solution components. Reaction thus prevented, it turns out that the polymer forms around the bioreactant with sufficient intimacy to replicate its shape, but without chemically attaching to it or embedding it (which would render it useless).
The bioreactant is thereby rendered accessible with full activity, unencumbered by steric hindrance from polymer embedment or undesirable reaction with functional groups. If desired, the bioreactant or biomolecule may even be extracted, the gel of phantoms then becoming a specific adsorbent for the biomolecule of interest. The enhanced affinity gel may also be used in combination with or coated over conventional solid adsorbents and substrates, such as activated carbon or reticulated foam.
Regardless of the application, of course, the gel must replicate the shape of the bioreactant and thus retain it while remaining porous. Porosity of the hydrogel is determined by the amount of water or other polymer nonsolvent present as the gel network is formed. If the water content is too high, the resulting gel will have poor mechanical properties or will not form at all; conversely, if the water content is too low, the gel will be insufficently porous. The best replicating gels for biological use generally contain between 20% and 80% by weight of water. For most such gels, approximately 50% to 60% water is preferred. Such a gel will readily permit passage of proteins and high organic toxins. Replication occurs as the polymer retracts while polymer cross-links develop and at least part of the non-solvent is expelled. Cross-linking by chain transfer is preferable in order to maintain intimacy with the bioreactant thus assuring replication. Exogenous cross linking, with a copolymer for example, should be kept below 5% and preferably in the range of 1% to 3%.